A EEMAEKABLE MAEINE ORGA^NIS^r. 449' 



species is said to be zoned in section, aad its surface is generally 

 mammillate." 



The resemblance o£ these calcareous pebbles to PontoloJhos 

 is certainly very striking, especially when we bear in mind that 

 the latter also contains a considerable quantity of carbonate of 

 lime. At the same time there appears to be no doubt that the 

 filaments of Pontoholhos do not belong to a Schizothrix. The 

 case o£ ScMzotlirix, however, seems to show that somewhat 

 similar filaments are capable by themselves of giving rise to a 

 massive laminated structure without the co-operation of any 

 symbiotic animal organism. 



We come now to the consideration of the second hypothesis 

 as to the nature of Pontohollos^ viz., that it is a symbiotic 

 organism, consisting of bacterial filaments growing in association 

 with a gigantic Ehizopod. This hypothesis would at once 

 explain the existence of the darkly staining flocculent layers 

 and network, which form by far the most conspicuous feature in 

 sections stained by any of the ordinary methods applicable to 

 animal tissues. Nor is it impossible to point to certain known 

 Ehizopods the structure of which seems to support this view. 

 I refer to some of the large Foraminifera, such as the extinct 

 Nummtilites, and especially to that remarkable fossil form 

 Loftusia persica, described by Carpenter and Brady *, which 

 attains a length of 3| inches and a breadth of 1| inches, and 

 exhibits a laminated appearance in section extremely suggestive 

 of Pontobolbos. 



In Loftusia, as in many Foraminifera, the typical calcareous 

 skeleton is replaced by sand. In many siliceous and horny 

 sponges a similar replacement takes place. Indeed, there seems 

 to be a strong tendency, both amongst Protozoa and Sponges, to 

 replace the proper skeleton with foreign material whenever such 

 material is available. They do this in accordance with what 

 might be termed " The Law of Economy in Skeleton-formation." 



It is also well known that symbiotic (or parasitic ?) Schizo-^ 

 phytes are frequently met with in large numbers in Sponges, 

 some of which, indeed, appear to be habitually infested with 

 such forms. Thus Schulze has described* the occurrence of 

 Oscillaria sjpongelice in Spongelia pallescens, where it occurs even 



* Phil. Trana. Eoyal Soc. 1869, p. 739 (Plates Ixxvii. to Ixxx.). 

 t Zeit. wiss., Zool. Bd. xxxii. p. 147. 



